Add MiniMax provider preset

[octo-patch]

Reason: Add MiniMax as a first-class provider for its OpenAI-compatible API and current model.

Changes:

• Added the MiniMax provider preset with the MiniMax-M3 default model, global endpoint, and documented China-region endpoint.
• Updated provider documentation and coverage tests.

Checks:

• npm test
• npm run check
• git diff --check